One hundred and twelve fluorescent Pseudomonas isolates from potato periderm were screened for antagonistic activity in vitro against 5 pathogenic and saprophytic fungi, 5 Gram‐positive bacteria including Streptomyces scabies and 4 Gram‐negative bacteria including Erwinia car. var. carotovora and Erwinia car. var. atroseptica. Fifty‐seven percent of the isolates showed wide spectrum inhibitory activity, mostly due to the production of fluorescent siderophores. Only 4 % of the isolates were very strong antagonists. By adding iron (Fe3+) to the test medium, the antagonistic activity drastically diminished but revealed some strong antagonists of which the antagonism was not affected. Apparently growth‐inhibiting substances other than siderophores were involved. Antibiotic‐resistant mutants of the antagonists were used to establish that the bacteria on the seed tubers migrated to the roots under different conditions. Drying of the treated potatoes gradually diminished the number of viable cells, but subsequent planting in moist soil led to substantial increases in the first three days. Initially high numbers of the antibiotic‐resistant antagonists on the roots tended to drop quickly in time, probably due to loss of resistance to the antibiotics in the selective medium.

Severe yield depressions observed in high frequency potato cropping caused by microbial factors other than eelworms or known pathogens (narrow rotation effect) were reproduced in pot experiments. Total plant weight in a soil continuously cropped with potatoes was only 69% of that in a similar soil continuously cropped with wheat. Tuber weight in potato‐soil was only 13 % and number of tubers only 10% of that in wheat‐soil. Seed tuber treatment with cell suspensions of fluorescent antagonistic pseudomonads in carboxymethylcellulose (CMC) raised tuber yield in potato‐soil up to 70% (550 % increase), number of tubers up to 93% (934 % increase) and total plant weight up to 90% (31 % increase) as compared with CMC‐treatment in wheat‐soil. Antagonistic fluorescent Pseudomonas isolates WCS 365, WCS 358 and WCS 374 producing mainly siderophores, with little or no production of other inhibitory substances, were more effective in diminishing the narrow rotation effect than the isolates that produced mainly inhibitory substances of different nature.

As an approach to understanding the molecular basis of the reduction in plant yield depression by root-colonizing Pseudomonas spp. and especially of the role of the bacterial cell surfaces in this process, we characterized 30 plant-root-colonizing Pseudomonas spp. with respect to siderophore production, antagonistic activity, plasmid content, and sodium dodecyl sulphate-polyacrylamide gel electrophoresis patterns of their cell envelope proteins. The results showed that all strains produce hydroxamate-type siderophores which, because of the correlation with Fe3+ limitation, are thought to be the major factor responsible for antagonistic activity. Siderophore-negative mutants of two strains had a strongly decreased antagonistic activity. Five strains maintained their antagonistic activity under conditions of iron excess. Analysis of cell envelope protein patterns of cells grown in excess Fe3+ showed that most strains differed from each other, although two classes of similar or identical strains were found. In one case such a class was subdivided on the basis of the patterns of proteins derepressed by iron limitation. Small plasmids were not detected in any of the strains, and only one of the four tested strains contained a large plasmid. Therefore, it is unlikely that the Fe3+ uptake system of the antagonistic strains is usually plasmid encoded.

The nematode Steinemema feltiae (Nematoda: Steinernematidae) was tested for its ability to control two main mushroom pests i.e. the sciarid Lycoriella auripila (Diptera: Sciaridae) and the phorid Megaselia halterata (Diptera: Phoridae) in growing-rooms filled with spawned compost. A clear difference between female and male sciarid control was observed. A nematode application 1 day after casing preceded by an application 1 day before casing on the compost caused an almost complete control (97%) of the F1-generation of female sciarids. The FZgeneration of females was similarly controlled (95%) by an application 7 days after casing. A dosage of 1 x lo6 nematodes mP2 was found to be equally effective as higher dosages. Diflubenzuron remained active throughout entire the cropping period with high sciarid mortality rates varying from 72% to 99%.Phorid control was variable and seemed to depend on the presence of sciarids. In one occasion the control rate of F2-generation phorid larvae was 75% and was possibly caused by the presence of new infective juvenile nematodes recycled in FZgeneration sciarid larvae. Diflubenzuron did not significantly reduce phorid numbers.

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